Optical fiber gratings are formed by exposing a photosensitive fiber such as, for example, boron-doped germanosilicate fiber to ultraviolet light so as to create permanent refractive-index perturbations at selective sections along the core of the fiber. A grating is a wavelength-selective reflector having a reflectance response curve with at least one well-defined peak. In other words, a fiber grating reflects light of a particular wavelength or a narrow band of wavelengths back along the original propagation direction, while permitting other wavelengths of light to propagate undisturbed. The reflected wavelength of light is often referred to as the grating wavelength.
Optical fiber gratings may be used in Wavelength Division Multiplexing optical systems for high-precision selective wavelength filtering so that signals propagating through an optical fiber can be separated, combined, and/or rerouted. They can also be used as feedback elements for a fiber optic laser or as external laser mirrors. For all such uses, it is essential that the grating wavelength remains constant over an expected temperature range of, for example, from -20.degree. C. to 80.degree. C. (i.e. .DELTA.T=100.degree. C.).
The grating wavelength .lambda..sub.g (or Bragg wavelength) is related to the effective guided mode index, n, of the optical fiber and the spacing of the grating sections, .LAMBDA. (also known as the grating period), in the following way: EQU .lambda..sub.g =2n.LAMBDA.,
This equation shows that the effective guided mode index, n, and the grating period, .LAMBDA. are inversely proportional to each other. Therefore, in order to maintain .lambda..sub.g constant, an increase, for example, in the effective guided mode index, n, requires a proportionate decrease in the grating period, .LAMBDA..
For optical communication systems, it is essential that the grating wavelength remains constant over the expected temperature range. But this requirement is not so easily satisfied since the effective guided mode index of a fiber varies rather significantly over an expected temperature range of, for example, from -20.degree. C. to 800.degree. C. (i.e. .DELTA.T=100.degree. C.), primarily due to the temperature dependence of the fiber's refractive index. It has been reported that over this temperature range, the grating wavelength shift of an uncompensated 1550 nm grating can exceed 1 nm, which can be detrimental to an optical communication system.
Fortunately, it can be readily shown that in order to hold .lambda..sub.g constant over a temperature range, an increase in temperature must be accompanied by a corresponding decrease in strain in the fiber grating and vice versa. Stated in a different way, the change in strain (.DELTA..epsilon.) and change in temperature (.DELTA.T) in a fiber grating are inversely and linearly related to each other that is: EQU .DELTA..epsilon./.DELTA.T=constant&lt;0
Accordingly, to compensate for or counteract an unwanted shift in grating wavelength, one could vary the grating period, .LAMBDA., through selective adjustment of strain in the fiber. Thus, for example, when the ambient temperature of the fiber grating rises, one may decrease the strain in the grating to maintain the same grating wavelength as that was set at the initial temperature condition. Similarly, when the ambient temperature of the fiber grating decreases, one needs to increase the strain in the grating so as to maintain the grating wavelength constant.
Heretofore, known temperature compensating devices for fiber gratings employ two expansion components mounted in series relative to the fiber grating, i.e., one end of the fiber grating is attached to the component disposed upstream of the fiber grating and the other end of the fiber grating is attached to the component disposed downstream of the fiber grating. The expansion components have different coefficients of thermal expansion. The fiber grating mounted between the two components is pre-strained so that an increase in the ambient temperature causes the two series-mounted components to lessen the pre-strain in the grating to compensate for the temperature dependence of the fiber grating and vice versa.
These known temperature compensating devices are relatively long in comparison with the length of the fiber grating. Accordingly, it is desired to provide a more compact temperature compensating device which offers significant space savings to the users.